raised checkpoint limit & manual checkpoint

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Cc: Andres Freund <andres(at)anarazel(dot)de>
Subject: raised checkpoint limit & manual checkpoint
Date: 2016-09-24 06:42:16
Message-ID: alpine.DEB.2.20.1609240818210.6473@lancre
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Hello,

The checkpoint time limit has just been raised to one day after a
discussion started by Andres Freund:

https://www.postgresql.org/message-id/20160202001320.GP8743%40awork2.anarazel.de

I would have gone further up, say one week or even one month, but I think
that this new limit is an improvement over the previous 1 hour maximum.

Now ISTM that there is a possible use case which arises with this new
setting and is not well addressed by postgresql:

Let us say that an application has periods of high and low usage, say over
a day, so that I want to avoid a checkpoint from 8 to 20, but I'm okay
after that. I could raise the size and time limits so that they do not
occur during these hours and plan to do a manual CHECKPOINT once a day
when I see fit.

Now the problem I see is that CHECKPOINT means "do a CHECKPOINT right now
as fast as possible", i.e. there is no throttling whatsoever, which leads
to heavy IO and may result in a very unresponsive database.

I would suggest that a good complementary feature would be to allow a
manual checkpoint to run over a period of time, say something like:

CHECKPOINT OVER '10 hours';

That would target to complete after this period (whether it succeeds or
not is another issue) instead of going as fast as possible, thus avoiding
some performance degradation.

Any thoughts?

--
Fabien.

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